146. P. Müller, H. Imogai,
     “On the Enantioselectivity of a 2-Aza-divinylcyclopropane Rearrangement”
     Helv. Chim. Acta 1999, 82, 315-322.

The optically active cyclopropanecarbonyl azide 6 (ee 90%) was synthesized from the known cyclopropanecarboxylate 4. Upon heating in refluxing benzene, 6 rearranged to the dihydroazepinone 9 (ee 65%) via the isocyanate 7 and the enamide 8. In contrast, the dimethyl-substituted cyclopropanecarbonyl azide 20 rearranged to the isocyanate 21, but decomposed at higher temperatures rather than to undergo Cope rearrangement to 22. The results are consistent with a mechanism involving a boat-like endo-transition state 11 for the rearrangement, which may not be reached with the dimethyl derivative 21 for steric reasons. The origin of change in enantiomer composition, upon going from 6 to 9, is at present unknown.

DOI : 10.1002/(SICI)1522-2675(19990310)82:3<315::AID-HLCA315>3.0.CO;2-T 

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